Petrography of Felsic Volcanic and Pyroclastic Rocks at Dam Embankment of Khun Dan Prakan Chon Dam,

Nakhon Nayok Province Department of Geological Sciences, Faculty of Science, Chiang Mai University Ms. Phetcharaporn Khueanpun Student ID: 590510389 Advisor: Dr. Patcharin KOSUWAN JUNDEE

The study site is located at the dam embankment of the Khun Dan Prakan Chon dam in Nakhon Nayok Province where the rocks consist of volcanic and pyroclastic rocks. The aims of this study are to focus on the lithology and petrography study of felsic volcanic and pyroclastic rocks, to classify rock types, using their color, mineral composition, rock component, texture and alteration, and to explain the volcanism of this area. These rocks are part of Loei-Phetchabun-Nakhon Nayok volcanic belts. The volcanic rocks are felsic and mafic rocks with porphyritic texture. The felsic volcanic rocks are rhyolite with alkali feldspar, plagioclase and quartz phenocrysts. The mafic volcanic rocks are shallow intrusion of basalt with plagioclase phenocrysts. Their groundmass consists of fine-grained crystals and glasses. The pyroclastic rocks are felsic-intermediate ash tuff, lapilli tuff and pyroclastic breccia which their components are lithic, crystal and minor vitric fragments. The lithic fragments are rhyolite, trachyte and basalt, crystal fragments are plagioclase, alkali feldspar and quartz, and vitric fragments are volcanic glasses. The matrix is fine ash of crystals, glasses and clay minerals. Alteration rate is typically moderate to high. Alteration products are sericite, chlorite, epidote, and calcite. From field observation and the petrographic study, the association of shallow mafic intrusion in massive rhyolite and rhyolitic pyroclastic rocks suggest the bimodal volcanism which characterizes an extensional environment.

Abstract

The study site is located in Khun Dan Prakan Chon dam Tambon Hin Tung, Mueang Nakhon Nayok District, Nakhon Nayok Province. Geologic map shows in figure1. There are 3 rock units in this area, Permo-Triassic volcanic rocks, Jurassic sedimentary rocks and Quaternary sediments. The 20 rock samples were collected along the transect of dam embankment for petrologic studied.

Introduction Study site

1 .To study lithology and petrography of felsic volcanic and pyroclastic rocks at dam embankment of Khun Dan Prakan Chon Dam

2. To characterize and classify rock type in this area

Objective

PTrv1

PTrv

Jpk

Qc Terrace gravel sand, silt and clay

Siltstone, reddish purple, calcareous, micaceous, sandstone: grayish green and calcrete conglomerate

Rhyolite, andesite, rhyolitic and andesitic tuff, agglomerate and volcanic breccia

Tuff, fine ash tuff

Figure 1. Location of study site (modified from Geologic map pf Nakhon Nayok, DMR 2519)

1. Field observation for collecting 41 samples from outcrops

2. Select 20 samples of felsic rocks for Lithology studying

3. Thin section preparation and Petrography study: to study texture, mineral composition, rock component and classify rock type

Methodology

The petrographic investigation of the rock at the dam embankment of Khun Dan Prakan Chon dam is the volcanic and pyroclastic rocks. The volcanic rocks are rhyolite and basalt. The pyroclastic rocks with rhyolitic composition are tuff, lapilli tuff and pyroclastic breccia which their fragment components are mostly lithic fragments of rhyolite and/or basalt. The field investigation shows that mafic volcanic rock (basalt) occurs as shallow intrusive in rhyolitic pyroclastic rock

Conclusion

Result

The study was supported by Department of Geological Sciences, Faculty of Science. Chiang Mai University

Acknowledgements

Afs

Qz

Afs

Qz

Pl

Pre

Pl

Pl

Pre

Pl

(a) (b)

Figure 2. Rhyolite (KD31) under polarizing microscope 4X (a) plane polarized (b) cross polarized

Figure 3. Basalt (KD29) under polarizing microscope 4X (a) plane polarized (b) cross polarized

(a) (b)

Pl

Pl

Rf Pl

Pl

Rf

Pl

Pl

Pl

Pl

Figure 4. Rhyolitic tuff (KD40) under polarizing microscope 4X (a) plane polarized (b) cross polarized

Figure 5. Trachytic tuff (KD11) under polarizing microscope 4X (a) plane polarized (b) cross polarized

References Hinthong, C. 1981. Geology and Mineral Resources of the Map Sheet Changwat Phranakorn Sri Ayutthaya (Map ND 47-8).

Geological Survey Report no. 4, Department of Mineral Resources, Bangkok, Thailand.

Salyapongse, S., Fontaine, H., Putthapiban, P. and Lamjuan, A. 1997. Geology of the eastern Thailand: Field excursion

guidebook. Geological Survey Division, Department of Mineral Resources, Bangkok.

(a) (b)

(b) (a)